Printers form ink images on media, which include paper and other print media. Different imaging or printing techniques, which include laser printing, inkjet printing, offset printing, dye-sublimation printing, thermal printing, and the like, can be used to produce printed documents. In particular, inkjet printers eject liquid ink from printheads to form images on an image receiving member surface. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member surface to form an ink image that corresponds to the digital data for the images used to generate the firing signals. The size of the ink drops and the timing of the ejection of the ink drops are affected by the frequency and amplitude of the firing signals.
Throughout the life cycle of a printer, the image generating ability of the device requires evaluation and, if the images contain detectable defects, correction. Various defects in the image generating process affect ink image quality. In an inkjet printer, one such defect occurs when an individual inkjet becomes inoperable as either a “weak” or “missing” inkjet. A weak inkjet intermittently ejects ink drops or ejects ink drops having a mass that is different than expected for the firing signal used to operate the actuator for the inkjet. A missing inkjet fails to eject ink drops entirely. Inoperable inkjets, including both weak and missing inkjets, negatively impact the quality of printed images.
Some existing printers are configured to detect and compensate for inoperable inkjets. Identifying inoperable inkjets typically requires the printing of reference patterns, which are specially designed, arranged ink lines printed on the image receiving member surface. These reference patterns are printed separately from the ink images forming a print job. Consequently, the printing of reference patterns absorbs a portion of the resources used for productive printing. Because a printhead often includes hundreds or thousands of individual inkjets, correct identification of a single inoperable inkjet presents challenges. In some imaging devices, an optical sensor is used to generate image data of the reference pattern on an image receiving member surface and these data are analyzed and correlated to inkjet positions in a printhead to identify an inoperable inkjet. Errors in the alignment of the photosensors in the optical sensor or in the calibration of the sensor along with distortions that arise from media shifting during operation of the printer affect the accuracy of the analysis of the image data of the reference pattern. In a situation where a printer incorrectly identifies an inoperable inkjet, the printer operates inkjets that are located near the identified inkjet to compensate for a perceived image defect produced by an inoperable inkjet. Compensating for the incorrectly identified inkjet, however, introduces image defects that compound the defects produced by the inoperable inkjets. Consequently, improvements to the identification and compensation for inoperable inkjets in an inkjet printer would be beneficial.